Excision sequences

Amplification of genes under selective conditions has been widely observed-for example, in development of pesticide-resistant forms of insects. Such amplified structures could arise either through recombination with unequal sister-chromatid exchange, schematized in Figure 25.41, or by a conservative transposition process. Later, homologous recombination within an amplified region can lead to excision of sequences containing one or more amplified sequences. In order to replicate autonomously, these excised sequences must have a centromere. Such elements probably represent the double-minute chromosomes. 

Figure 2.12. Primary structure of the ori sequences and their flanking regions. Regions of ori sequences are indicated by thick lines for GC clusters A, B and C and thin lines for AT stretches p, s, I and r. The positions of GC clusters a in ori 4, p in on 4 and 6, y in ori 4, 6 and 7, and S in on 1 are given r sequences are indicated by heavy line boxes r and r sequences by broken line boxes. Other boxes indicate sequences /a, Ip, /g, rz, the excision. sequences of the repeat units of petites a-l/lR/1/26, a-l/lR/14, a-3/1/33, a-3/1/5, anda-l/IR/Z] and the initiation triplet of the open reading frames of ori 1, 2, 3 and 5. (From de Zamaroczy et al., 1983).

Gaillard C., Strauss F., Bernardi G. (1980). Excision sequences in the mitochondrial genome of yeast. Nature 283 218-220. 

Note that the key to Maxam-Gilbert sequencing is to modify a base chemically so that it is removed from its sugar. Then piperidine excises the sugar from its 5 - and 3 -links in a /3-elimination reaction. The conditions of chemical cleavage described in Figures 12.4 and 12.5 are generally adjusted so that,... 

Alfuzosin (91) is a prazosin-like hypotensive adrenergic a-1 receptor blocker with the special structural feature that two carbons have been excised conceptually from the piperazine ring normally present in this series. Following the usual sequence for this series, reaction of 4-amino-2-chloro-7-dimethoxyquinazoline (89) with the tetrahydro-2-furyl amide of 3-methylaminopropyla-mine (90) gives alfuzosin (91) [25], Alfuzosin is claimed to cause less orthostatic hypotention (dizziness or fainting upon sudden rising) than prazosin. 

Splicing is a processing step of the pre-mRNA to become a mature transcript. This involves the excision of intervening noncoding sequences (introns) from coding sequences (exons) by a multiple protein complex, the spliceosome. After splicing the mRNA molecule is ready for translation, since it contains a continuous sequence that encode an entire protein. 

Classical global knockouts may have a developmental or lethal phenotype and thus preclude the analysis of the phenotypic consequences of the lack of a gene in specific tissues in adult animals. With the development of the cre/loxP and flp/FRT systems, it has become possible to excise defined DNA fragments from the genome of specified cells. Cre and Flp are bacterial and yeast recombinases, respectively, which recognize loxP and FRT sequences, respectively. The most common... 

Intron The sequence of a gene that is transcribed but excised before translation. 

Figure 4.5. Integrative and excisive X phage recombination pathways. Integration is catalyzed by the X Int protein in a reaction that also requires the E. coli IHF protein. Recombination occurs within a common core sequence of 15 base pairs. The excision reaction requires the X Xis protein in addition to Int and IHF.

The reaction differs from excision of the X chromosome because the Entry Clone contains two attL sites and the destination vector contains two attR sites (Hartley et al., 2000). The att sites are mutated to ensure recombination only occurs between attLl and attRl and between attL2 and att.R2. The recombination reaction proceeds through a cointegrate molecule that is resolved to create a destination vector containing the gene of interest with the desired promoter and tag sequences (Fig. 4.6). 

D Ham, C. Romieu, A. Jaquinod, M. Gasparutto, D. Cadet, J. Excision of 5,6-dihydroxy-5,6-dihydrothymine, 5,6-dihydrothymine, and 5- hydroxycytosine from defined sequence oligonucleotides by Escherichia coli endonuclease III and Fpg proteins Kinetic and mechanistic aspects. Biochemistry 1999, 38, 3335-3344. 

Figure 17.26 The native process leading to intein excision and ligation of extein fragments involves a sequence of reactions involving transthioesterification, cleavage of the intein fragment, and aS->N shift, which ligates the two extein peptides together via an amide bond.

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